feat: implement deep sleep for battery operation (#5)

examples/deep_sleep/README.md

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+# Deep Sleep Example
+
+This example demonstrates ESP32 deep sleep for battery-powered MicroClaw operation.
+
+## Features
+
+- **Ultra-low power**: ~30µA in deep sleep (vs ~160mA awake)
+- **Timer wakeup**: Configurable interval (default 5 minutes)
+- **Battery monitoring**: ADC-based voltage reading with low-battery protection
+- **Fast boot**: WiFi reconnect in ~2 seconds
+- **MQTT reporting**: Publish sensor data, then sleep
+- **Boot counter**: RTC memory persists across sleep cycles
+
+## Expected Battery Life
+
+With 1000mAh LiPo battery:
+
+| Sleep Interval | Average Current | Battery Life |
+|----------------|-----------------|--------------|
+| 5 minutes      | 2.65 mA         | 15.7 days    |
+| 15 minutes     | 0.95 mA         | 1.5 months   |
+| 1 hour         | 0.25 mA         | 5.5 months   |
+
+See `deep_sleep.ino` for detailed calculation.
+
+## Wiring
+
+### Battery Connection
+
+```
+LiPo 3.7V (1000-2000mAh)
+   │
+   ├─ ESP32 3.3V pin (or VIN with voltage regulator)
+   │
+   └─ Voltage divider for ADC:
+      Battery+ ──[100kΩ]──+──[100kΩ]── GND
+                          │
+                       GPIO 35 (ADC)
+```
+
+### Voltage Divider Calculation
+
+```
+Battery: 4.2V max (fully charged LiPo)
+ADC max: 3.3V (ESP32 limit)
+
+With 2:1 divider (100kΩ + 100kΩ):
+  ADC voltage = Battery / 2
+  4.2V → 2.1V (safe for ESP32)
+  3.0V → 1.5V (low battery threshold)
+```
+
+## Configuration
+
+Edit `deep_sleep.ino` and change:
+
+```cpp
+const char* WIFI_SSID = "your_wifi_ssid";
+const char* WIFI_PASSWORD = "your_wifi_password";
+const char* MQTT_BROKER = "mqtt.example.com";
+
+#define SLEEP_INTERVAL_SECONDS 300  // 5 minutes
+#define LOW_BATTERY_THRESHOLD 3.2   // Volts
+```
+
+## Flashing
+
+```bash
+# Navigate to this directory
+cd examples/deep_sleep
+
+# Upload via PlatformIO
+pio run --target upload
+
+# Or use Arduino IDE:
+# 1. Open deep_sleep.ino
+# 2. Select board: ESP32 Dev Module
+# 3. Upload
+```
+
+## Monitoring
+
+```bash
+# Watch serial output (baud: 115200)
+pio device monitor
+
+# Or use screen:
+screen /dev/ttyUSB0 115200
+```
+
+Expected output:
+
+```
+========================================
+🐾 MicroClaw Deep Sleep Example
+   Boot #1
+========================================
+⏰ Wakeup: Unknown reason 0
+🔋 Battery: 3.85V
+📡 Connecting to WiFi... Connected!
+   IP: 192.168.1.100, RSSI: -52 dBm
+🔗 Connecting to MQTT... Connected!
+📤 Publishing sensor data...
+✅ Publish successful!
+   Data: {"boot_count":1,"temperature":25.0,...}
+
+💤 Entering deep sleep for 300 seconds...
+   Current draw: ~10-50µA (vs ~160mA awake)
+   Next wakeup: Timer
+```
+
+## Troubleshooting
+
+### ESP32 won't wake up
+
+**Cause**: EN pin pulled LOW externally  
+**Fix**: Remove external connections to EN pin
+
+### Battery drains quickly
+
+**Possible causes**:
+- WiFi takes too long to connect (>10s)
+- MQTT broker unreachable (hangs)
+- Sensors not powered down
+
+**Fixes**:
+- Reduce WiFi timeout to 10s
+- Add MQTT connection timeout
+- Turn off sensor power via GPIO before sleep:
+  ```cpp
+  digitalWrite(SENSOR_POWER_PIN, LOW);
+  ```
+
+### ADC reads incorrect voltage
+
+**Cause**: No voltage divider or wrong ratio  
+**Fix**: Use 2:1 divider (100kΩ + 100kΩ) and adjust multiplier:
+```cpp
+float voltage = (adcValue / 4095.0) * 3.3 * 2.0;  // *2.0 for 2:1 divider
+```
+
+### Boot loop (constant resets)
+
+**Cause**: Low battery voltage (brownout detector)  
+**Fix**: Charge battery or disable brownout:
+```cpp
+#include "soc/soc.h"
+#include "soc/rtc_cntl_reg.h"
+
+void setup() {
+    WRITE_PERI_REG(RTC_CNTL_BROWN_OUT_REG, 0);  // Disable brownout detector
+    // ... rest of setup
+}
+```
+
+## Power Optimization Tips
+
+### 1. Minimize Awake Time
+
+- Use static IP to skip DHCP (~1s saved)
+- Cache WiFi credentials (ESP32 does this automatically)
+- Disable mDNS/DNS lookups
+
+### 2. Reduce TX Power
+
+```cpp
+WiFi.setTxPower(WIFI_POWER_11dBm);  // Reduce from 20dBm (default)
+```
+
+### 3. External RTC Module
+
+Use DS3231 RTC to wake ESP32 via external interrupt (saves even more power).
+
+### 4. Sensor Power Control
+
+```cpp
+#define SENSOR_POWER_PIN 25
+
+pinMode(SENSOR_POWER_PIN, OUTPUT);
+digitalWrite(SENSOR_POWER_PIN, HIGH);  // Turn on sensors
+delay(500);  // Sensor warmup
+// ... read sensors
+digitalWrite(SENSOR_POWER_PIN, LOW);   // Turn off before sleep
+```
+
+## Next Steps
+
+- Integrate DHT22/DS18B20 sensor readings (see `../dht22_example/`)
+- Add external wakeup (GPIO button or PIR sensor)
+- Implement OTA updates (wake on demand)
+- Use RTC memory for configuration persistence
+
+## References
+
+- [ESP32 Deep Sleep API](https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/sleep_modes.html)
+- [ESP32 Power Consumption](https://lastminuteengineers.com/esp32-sleep-modes-power-consumption/)